It is desirable to achieve a flat frequency response in equipment used for reproduction of music. A flat frequency response facilitates a proper tonal balance (i.e., the balance or volume relationships between different regions of the frequency spectrum, including bass, lower midrange, midrange, upper midrange and highs).
Human hearing is non-linear. That is, humans perceive sound pressure levels differently for different frequencies. The non-linearity is also dependent on sound pressure levels. Therefore, a human hears a different tonal balance for the same piece of music when the volume level is changed. Generally speaking, at lower volume levels, low frequencies are quite attenuated and high frequencies are somewhat less attenuated.
It is known to provide a loudness button or control on an audio device such as an amplifier. The loudness control activates a filter lifting low and high frequencies to some fixed chosen loudness setting.
However, the nature of loudness is continuous, so any chosen setting is only accurate for a particular sound pressure level. Sound pressure level is dependent on a variety of factors, including volume setting, electronic attributes, recording content and level, and so forth.